Most modern aircraft are powered by gas turbine engines, also known as jet engines. There are several types of jet engines, but all jet engine propulsion systems have certain parts in common. For example, all jet engine propulsion systems have an inlet with which to bring in free stream air into the engine. The inlet sits upstream of the compressor and, while the inlet does no work on the flow, there are important design features associated with the inlet. The total pressure through the inlet changes because of several flow effects. The inlet pressure performance is often characterized by the inlet pressure recovery, which measures the amount of free stream flow conditions that are recovered. This pressure recovery depends on a wide variety of factors, including inlet shape, aircraft speed, air flow demand of the engine, and aircraft maneuvers. The above effects are not uniform over the face of the engine.
Flow field vortices generated by fluid flow over aerodynamic surfaces within the inlet can cause buffet and fatigue any downstream structure exposed to these vortices and reduce engine performance. Vortices can be generated at the fore body of an aircraft or other upstream structure, and damage control surfaces, engines, after body/empennage, nacelles, turrets, or other structures integrated into the airframe. Additionally, these vortices can be ingested within engine air intakes or other like air inlets leading to poor performance, excessive blade vibration, and/or stalling of the aircraft engines. Stalling the aircraft engine and/or excessive blade vibration create a potentially hazardous conditions.
Next generation aircraft, such as blended wing body, compound this problem by incorporating gas turbine inlets with serpentine flow paths within the air frame. Additionally, exotic aperture shapes for the inlet and outlet may cause excessive propulsion performance losses. These losses emanate from strong secondary flow gradients in the near wall boundary of the airflow, which produce coherent large-scale vortices.
Compressor face distortion can lead to high amplitude circumferential harmonics at critical engine speeds causing excessive vibration of fan or compressor blades, leading to blade failure due to high cycle fatigue. In the past, such problems have been solved by redesign of the inlet duct or redesign of the fan or compressor blades by adding dampening or increasing blade strength to change the natural frequency. Any of these changes may involve increased cost and weight associated with the aircraft.
Another solution employs passive vortex generator vanes to mitigate the effects of flow field vortices. However, these vanes result in increased weight and reduced performance over the entire operating envelope of a vehicle. Vortex generators are small wing like sections mounted on an aerodynamic surface exposed to the fluid flow and inclined at an angle to the fluid flow to shed the vortices. The height chosen for the best interaction between the boundary layer and the vortex generator is usually the boundary layer thickness. The principle of boundary layer control by vortex generation relies on induced mixing between the primary fluid flow and the secondary fluid flow. The mixing is promoted by vortices trailing longitudinally near the edge of the boundary layer. Fluid particles with high momentum in the stream direction are swept along helical paths toward the duct surface to mix with and, to some extent replace low momentum boundary layer flow. This is a continuous process that provides a source to counter the natural growth of the boundary layer creating adverse pressure gradients and low energy secondary flow accumulation.
The use of vortex generators to reduce distortion and improve total pressure recovery has been applied routinely. Small-geometry surface configurations affect turbulent flow at the boundary layers.
Pressure recovery and distortion at the engine face within the inlet depend on a wide variety of factors, including the shape of the inlet, speed of the aircraft, air flow demands, and aircraft maneuvers. Since a variety of factors effect pressure recovery and distortion of the inlet, the airflow may not best served by a passive flow control. Solutions such as passive vortex generators, which reduce distortion and improve total pressure recovery, are optimized for certain operating conditions of the aircraft. As the aircraft may maneuver and engine air flow requirements may change, a single solution is not best suited to improve the pressure recovery and distortion of the engine inlet over the operating envelope of the aircraft.